Nice. But I think you want a) the starttime to be midnight and b) the
durations of the static images to be 2700 and the transitions to be 900,
so that it actually sums up to 24 hours.

Yeh, I screwed it up. I wasn't sure what exactly the relationships of
the numbers were and I didn't realize until I already built the package.
To be midnight the starttime's hour would have to be 0, so 12 is
actually 12 pm?

Does it matter what year-month-date is there?

I did each one using gradients and layer blending modes in the Gimp over
top of the main background image. The source is here (warning: it's
quite large):

Yeh. Because it's a significant amount of effort to produce them (this
set took over 4 hours of manual tweaking), I figured these are a good
first cut to make sure the colors work out and to get more eyes on the
background transition stuff. In time for test 3, I can go in
layer-by-layer (because each image is actually an alpha-gradient
adjustment layer over the original artwork in a gimp file) and manually
smooth each gradient. I thought that would be a significant amount of
time and effort to do when I wasn't sure I picked the right colors and I
really wanted something in test 2... does that make sense?

(I already ended up changing colors a few times to make the transitions
more smooth / colors more natural when I showed this around to people,
and I expect to get more feedback once more people try them out. I hope
this was an ok approach?)